A conventional heat engine, such as a Stirling engine, is able to produce power from an external heat source. The heat source may be low cost or free, such as waste heat from a manufacturing process, but the Stirling cycle itself requires a heavy and expensive engine resulting in prohibitive cost and weight for many applications. Another limitation of the Stirling engine is that working fluid must be heated and cooled with every cycle of the engine. This limits the speed at which the engine can operate and requires sophisticated heat exchangers.
Other conventional heat engines, such as steam engines that use the Rankine cycle, require the working fluid in the engine to change phases during operation of the engine. For a particular operating fluid, such as water, the engine requires a heat source at a temperature at over the boiling point of the operating fluid, 100 degrees Celsius for water, in order for the fluid to change to steam during the Rankine cycle. The Rankine cycle is not easily implemented in applications where the temperature at the heat source changes over time or where only small variations in temperature are available to operate the engine.
The inventor has previously proposed a cold cycle engine in PCT application no. PCT/CA2008/001149, published as no. WO2008/154730 published Dec. 24, 2008 and in US published application no. 2009-0165461 published Jul. 2, 2009. The cold cycle engine disclosed in those applications comprised passageways defining a path and containing a compressible fluid, which is at least in part pressurized above atmospheric pressure during normal operational conditions. The compressible fluid has a constant phase in the path. A pressure-displacement coupled interface is on the path and divides the path into a first energy transfer circuit and a second energy transfer circuit. In operation, the first energy transfer circuit and second energy transfer circuit have differential pressure, with one at higher pressure than the other. Depending on time of operation, this pressure differential may be reversed. First flow control devices on the first energy transfer circuit are coordinated to permit pulsed flow through the first energy transfer circuit with energy transfer through the pressure-displacement coupled interface. Second flow control devices on the second energy transfer circuit are coordinated to permit pulsed flow through the second energy transfer circuit with energy transfer through the pressure-displacement coupled interface. The first flow control devices and the second flow devices are coordinated to allow the pulsed flows in the first energy transfer circuit and the second energy transfer circuit to combine to create flow around the path. An input-output device is coupled to the pressure-displacement coupled interface to input energy into or extract energy from the pressure-displacement coupled interface.